1. Field of the Invention:
The present invention relates to a resistance welding electrode for use in welding metal plates made of aluminum, magnesium, or an alloy thereof.
2. Description of the Related Art:
For welding metal plates, superposed regions of the metal plates are inserted between a pair of confronting electrodes of a welding apparatus, the electrodes are pressed against the superposed regions of the metal plates, and an electric energy is supplied through the electrodes to the superposed regions of the metal plates to heat the superposed regions due to the electric resistance of the metal plates for thereby fusing the metal plates to each other. Heretofore, electrodes which have been used to resistance-weld relatively soft metal workpieces such as metal plates made of aluminum, magnesium, or an alloy thereof have spherical convex tip ends for preventing deformations or cracks from being developed around the welded regions of the workpieces.
When metal plates made of aluminum, magnesium, or an alloy thereof are resistance-welded by the conventional electrode, however, the electrode is locally overheated, and a mass of molten metal of the metal plates tends to adhere to the overheated region of the electrode. Another problem is that the overheated region of the electrode itself is liable to be broken or damaged. As a result, the electrode cannot be used continuously for a long period of time.
The foregoing problems encountered in resistance-welding metal plates of aluminum, magnesium, or their alloy with the conventional resistance welding electrodes are considered to be caused by the presence of oxide films on the surfaces of the metal plates. The oxide films are irregular films produced by oxidization in the air of the surface of the metal plates, and act as an insulation.
When the electrodes are pressed against the metal plates covered with the oxide films, cracks are developed in weak or fragile areas, such as thin areas, of the oxide films. Since a current is supplied through the cracks to the metal plates, the welding current concentrates in the cracks, excessively heating the cracked areas of the oxide films.
There have heretofore been proposed resistance welding electrodes having a surface for contacting a metal plate to be welded which is machined to break an oxide film on the metal plate. For example, Japanese laid-open patent publication No. 58-159986 discloses a resistance welding electrodes including a surface for contacting a metal plate to be welded, the surface having a plurality of concentric grooves or a spiral groove defined therein and a triangular or trapezoidal cross-sectional shape. The resistance welding electrode includes a plurality of concentric ridges or a spiral ridge of a sharp edge disposed between the grooves or turns of the groove. There have also been proposed resistance welding electrodes having on their tip end a plurality of independent protrusions each having a distal end thinner than its proximal end, as disclosed in Japanese laid-open patent publications Nos. 4-339573 and 5-92275.
When the disclosed resistance welding electrodes are pressed against a metal plate for resistance-welding the same, an oxide film on the metal plate is broken in an entire area which is contacted by the resistance welding electrode, and the resistance welding electrode can be held in direct contact with a non-oxidized area of the metal plate. Since a current is supplied from the contact area, the current does not concentrate on a local area, thereby avoiding excessive heating of the metal plate.
However, the disclosed resistance welding electrodes are disadvantageous in that since they leave a tool mark indicative of the shape of the tip end of the electrode on the welded area, the welded workpiece has a poor appearance, and stresses tend to concentrate on the marked area, resulting in sudden or fatigue-induced breakage. When welded metal plates are to be plated or painted, the plated or painted coating is apt to become irregular in the marked area because of surface irregularities of the tool mark.
Japanese laid-open patent publications Nos. 1-62287 and 3-230872 disclose resistance welding electrodes in which a material, such as ceramics, W, Mo, or the like which has an electric conductivity lower than that of an electrode matrix is exposed in an area of the tip end of the electrode which is susceptible to damage. The disclosed resistance welding electrode has its service life increased by protecting the vulnerable area with the exposed material which has an electric conductivity lower than that of the electrode matrix.
The resistance welding electrode disclosed in Japanese laid-open patent publication No. 1-62287 is primarily designed for use in resistance welding of galvanized steel sheets. The resistance welding electrode disclosed in Japanese laid-open patent publication No. 3-230872 is primarily designed for use in projection welding of galvanized steel sheets. In the disclosed resistance welding electrodes, the material which has an electric conductivity lower than that of the electrode matrix is exposed on the area which can be spotted in advance as being open to damage in those specific welding processes. Consequently, those disclosed resistance welding electrodes are not effective to prevent damage due to excessive heating of unspecific areas such as fragile areas of irregular oxide films on the surfaces of metal plates which are made of aluminum, magnesium, or an alloy thereof.